1. Field of the Invention
This invention relates generally to a free-piston Stirling engine driving a linear alternator to generate electrical power and more particularly relates to improvements in a closed loop, negative feedback control system for such an electrical power generating source.
2. Description of the Related Art
This invention is directed to an improvement in a control system for controlling a free-piston Stirling engine driving a linear alternator for converting heat energy to electrical power. The improvements include a novel control loop for controlling piston stroke to maintain the mechanical power generated by the engine equal to the power transferred from the engine to the linear alternator and also include a control loop for controlling another variable, such as DC output voltage or head temperature. Embodiments of the invention may also be applied to control multiple such engine/alternator pairs and maintain them in synchronization.
Control Circuits
As known to those skilled in the control system art, a closed loop, negative feedback control system has a forward loop and at least one feedback loop. The forward loop has a command input that is applied to a summing point (or summing junction). The command input is a signal representing a desired (commanded) value of an operating output variable parameter that is being controlled and provides a reference signal.
The forward loop also has at least one and may have a series of forward control elements (also known as “dynamic units”) that perform a mathematical operation on a signal that passes along the forward loop. Each forward control element has a forward transfer function which is a mathematical expression relating its input signal to its output signal.
A feedback loop of a closed loop, negative feedback control system has a sensor for measuring the actual value of the variable parameter that is being controlled and applies a signal representing that actual value to the summing point. The feedback loop may also perform one or more mathematical operations on the measured signal, such as scaling, before applying it to the summing point. The output of the summing point provides an error signal representing the difference between the desired value and the measured value of the controlled parameter. That error signal is applied to a forward control element.
Closed loop, negative feedback control systems are not limited to a single feedback loop, a single summing point or a single forward control element. A closed loop control system may have multiple summing points interposed between multiple forward control elements that are connected in series along the forward loop. Multiple feedback loops, each sensing and feeding back a signal representing a different sensed variable parameter, are connected to these summing points. Consequently, each summing point has an input representing a command input, an input representing a feedback signal that represents a sensed variable parameter and an output representing a difference between the inputs to the summing point. Each summing point also might have inputs representing disturbances or external force or torque loads, for example. Although the difference often represents the difference (an “error”) between a commanded value of an output variable and a sensed value of the same variable, the difference can also be simply a modification of one signal by another signal described mathematically. There are also other types of circuit connections, such as a feed forward loop. Elements of a control circuit are conventionally represented in a control circuit diagram as mathematical expressions for the operations they perform on their input signals. The mathematical expressions are advantageously Laplace transform expressions and tell an engineer skilled in the art the operating characteristics of the elements in the control system and therefore how to construct hardware implementations of them. Ordinarily, there are multiple computing circuits known to those skilled in the art to implement each element in the control system so long as they perform the transfer function described by their mathematical expressions.
Persons skilled in the control system art also recognize that such control systems can be implemented with analog or digital computing circuits and combinations of them. The mathematical operations described in the diagram of a control system are desirably implemented with any of a variety of commercially available microprocessors, microcontrollers or other computing circuits. As known in the current state of the art, analog circuit and mathematical operations can be economically performed by software programmed digital circuits having software algorithms that simulate analog circuit operations and perform or compute mathematical operations. Many of these operations can be performed by discrete logic, programmable logic array (PLA), programmable gate array (PGA) or digital signal processor (DSP) implementations, as well as by microprocessors or microcontrollers. Therefore, the terms “control circuit” and “controller circuit” generically include the known types of analog and digital logic control implementations that can be used to implement the control circuit illustrated on a control circuit diagram. The term “computing circuit” refers to circuit implementations utilizing such circuits for transforming an electrical signal in accordance with a mathematical operation or algorithm.
Free-Piston Stirling Engines and Alternators
A free-piston Stirling engine (FPSE) driving a linear alternator is an attractive electrical power source because such sources are efficient, compact and light weight and can generate electrical power from heat energy supplied by a variety of fuels. A free-piston Stirling engine is a closed-cycle, reversible heat engine which converts heat into work by moving a confined volume of working gas between a relatively warmer heat acceptor and a relatively cooler heat rejector. The resulting alternating, cyclical, expansion and compression of the internal working gas provides an oscillating pressure wave that drives an appropriately sprung piston to oscillate substantially sinusoidally in linear reciprocation. The piston is mechanically linked to a ring of permanent magnets that it drives in reciprocation within the winding or coil of the linear alternator thereby inducing a voltage across the winding terminals. Typically, the piston of the engine is directly linked by a flange on the back of the piston to an array of axisymmetrically arranged magnets, for example arranged in a ring, and the engine and alternator are integrated into a common, hermetically sealed housing.
Many prior art electrical power sources of this type include a rectifier circuit connecting the alternator output terminals to an electrical load and also have a controller which is a control system for controlling the operating parameters of the Stirling engine and the alternator as well as the output electrical parameters. The operation of a free-piston Stirling engine and its connection to a linear alternator are described in many publications, including patents such as U.S. Pat. No. 6,871,495 which is herein incorporated by reference.
Energy for driving the FPSE is supplied from an external heat source, such as fuel combustion, solar energy or heat from radioisotope power sources, applying heat to the engine heat acceptor (“hot end”). The heat energy is converted by the engine to mechanical work energy which drives the linear alternator to convert the mechanical energy to electrical energy. It is highly desirable that the mechanical power generated by the FPSE be exactly equal to the power transferred from the FPSE to the linear alternator, most of which is ultimately transferred to the load. This balanced power condition avoids significant problems with engine operation. If the power transferred to the alternator exceeds the power generated by the FPSE, the engine will stall. If the power transferred to the alternator is less than the power generated by the FPSE, the piston stroke will increase uncontrollably and can cause damaging internal collisions and engine temperature will slowly increase over time. Piston stroke is the distance traveled by the piston between the boundaries of its reciprocation. Piston motion as a function of time can be represented as a phasor with a piston amplitude XP and is sometimes alternatively used to describe piston displacement. Piston amplitude XP has a magnitude of one half piston stroke and the two terms are sometimes used interchangeably when describing qualitative aspects of operation.
Fuel combustion systems for electrical power generating systems of this type commonly have temperature control systems that control the temperature at the engine's heat acceptor. Consequently, for relatively long term control, the mechanical power delivered from the engine to the alternator can be modulated by increasing or decreasing the thermal input power to the engine head which causes its temperature to change. However, that is an insufficient control for at least two reasons. First, the rate of temperature change that can be accomplished is relatively slow, far too slow to respond in time to prevent either engine stalling or piston over-stroking. Second, engine efficiency is strongly dependent upon heat acceptor temperature. The hotter the heat acceptor temperature at the engine head, the more efficient the engine. Therefore, modulating thermal input power and temperature not only is too slow but also reduces engine efficiency because it does not maintain the highest possible input head temperature. It is therefore desirable to provide a way to control engine output power in order to maintain the power transfer balance between the engine and the alternator, but it is desirable to do so in a way that permits the hot end temperature of the FPSE to remain at a constant maximum temperature in order to maximize engine efficiency. Although embodiments of the invention advantageously include a fuel combustion control system that maintains a constant temperature that is as hot as the engine materials can withstand, that alone is not a viable option for controlling engine power output and piston stroke and maintaining the power balance described above.
Prior Art Examples
FIG. 1 is a simplified schematic diagram illustrating the electrical circuit of a prior art electrical power generating source having a linear alternator 10 driven by a free-piston Stirling engine and applying the alternator AC output to a load 12. The alternator is shown as its equivalent, lumped-element circuit. This equivalent circuit has, in series connection, an inductor 14 having inductance Lalt representing alternator winding inductance, a lumped resistance Rac representing alternator resistance and an AC voltage source 16 having an induced or back emf Vg. The voltage Vg is the open circuit voltage induced in the alternator winding by the magnets that are driven in reciprocation by the free-piston Stirling engine.
A tuning capacitor 18 is frequently connected in series with the alternator winding in order to tune out the winding inductance. The capacitance of the tuning capacitor is chosen so that, at the operating frequency of the alternator and engine, the inductive reactance of the winding and the capacitive reactance of the tuning capacitor form a series resonant circuit. Such series resonant circuits exhibit a zero or resistive impedance. The tuning capacitor consequently provides a unity or near unity power factor which maximizes power transfer from the alternator to the electrical load and minimizes resistive heat losses. However, such tuning capacitors are bulky and expensive so it is desirable to eliminate the tuning capacitor. Also, the tuning capacitor impedance and the series inductor impedance only match at a single frequency. Therefore, with a tuning capacitor, the output power factor varies with engine operating frequency. The controller of the present invention can compensate over a wide range of frequencies, provide unity power factor or constant frequency operation.
As also illustrated in FIG. 1, the output of the alternator can be connected to the utility electrical power grid 20 and used to supply electrical power to the grid. As known to those skilled in the prior art, if a tuning capacitor 18 is used to balance or cancel the inductive reactance of the alternator winding, this arrangement results in the free-piston Stirling engine operating at the same frequency and substantially in phase with the electrical grid. This synchronous operation occurs because the Stirling engine is coupled to the alternator by the magnetic coupling between the reciprocating permanent magnets and the alternator winding. The coupling of the magnetic flux of the reciprocating magnets with the magnetic flux resulting from the alternator current, causes the alternator current to be reflected into the engine as complex damping forces acting upon the free-piston Stirling engine. These forces, which are reflected back into the engine, act upon the piston of the engine as a combination of mass, spring and damping forces. If a tuning capacitor is used, the magnetic force generated by the alternator current acts upon the Stirling engine piston to cause the piston to run synchronously with the alternator current in the same way that a rotating synchronous electric motor, having two rotating magnetic fields, remains synchronous. If the magnetic field from the reciprocating magnets becomes advanced or retarded from the magnetic field from the alternator current, there is a magnetic force pulling them together. In the linearly reciprocating engine and alternator, the result is that the reciprocating piston of the FPSE will operate synchronously with the electrical power grid voltage, if the FPSE is designed to be mechanically resonant at or very near the power grid frequency and the capacitor is tuned for series resonance with the alternator winding.
FIG. 2 illustrates an electrical power generating source like that of FIG. 1 and known in the prior art, but having a common, passive, full wave rectifier 22 using four diodes arranged in an H-bridge to provide a DC output. The prior art has also substituted a full wave, switching mode rectifier, also known as an active rectifier, for the full wave diode rectifier of FIG. 2 and eliminated the tuning capacitor 24 by various techniques. An example of such a configuration is shown in the above cited U.S. Pat. No. 6,871,495.
Switching Mode Rectifiers
A switching mode rectifier is a type of circuit that is known in the prior art and described in multiple publications. It typically has an H-bridge configuration but has controllable electronic switches, commonly MOSFETs, substituted for the diodes of FIG. 2. An active rectifier controller or control circuit is connected to the gate of each electronic switch and switches them ON and OFF by switching one diagonally opposite pair ON and the other pair OFF and alternating the pair that is ON while the other pair is OFF. This switching is done at a frequency that is much higher than the sinusoidal frequency of the FPSE and alternator. For example, the electronic switches may be switched at a rate of 10 kHz or 20 kHz while the FPSE and alternator may be operated at 60 Hz or 120 Hz. The switching control not only turns the electronic switches ON and OFF as described, but also varies the duty cycle of the electronic switches in response to a modulating input signal. The switching control of a switching mode rectifier is essentially a pulse width modulator that includes a high frequency oscillator for alternately switching the diagonally opposite switch pairs and also modulates the duty cycle of the ON and OFF switching states that are switched at the high frequency. The phase of the switching is a function of the phase of the signal that control the pulse width modulator and the duty cycle of the switch pairs is a function of the amplitude of that control signal. As a result, the phase of the switching of the switching mode rectifier controls the phase of the current though the H-bridge relative to the alternator terminal voltage. However, because the phase control by the switching mode rectifier does not depend upon resonance, which is frequency dependent, a switching mode rectifier can maintain a desired phase relationship over a range of engine operating frequencies. The pulse width modulating circuit and function can be implemented not only with analog circuits but also and more importantly using microprocessors or microcontrollers, as is preferred, and other digital logic and processing circuits that are programmed, such as with software, to perform the pulse width modulating function. Because switching mode rectifiers, summarized above, are described in prior art text books and technical literature about switch mode power supplies, switch mode inverters or switch mode motor drives, switching mode rectifiers are not explained here in more detail.
Variations
FIG. 1 also illustrates the use of a “dump” resistance 26 as another prior art way of controlling piston stroke and maintaining the power balance between the engine and the alternator. The resistance 26 is an additional electrical load that can be switched into the circuit or varied in resistance to essentially waste excess power produced by the engine. However, this is obviously undesirable because it simply dissipates excess power produced by the engine in order to maintain the power balance and consequently reduces efficiency by wasting heat energy and therefore wasting fuel.
The prior art has recognized that the power out from a FPSE can be controlled by controlling piston stroke because the power produced by a FPSE is approximately proportional to the square of the piston stroke. However, the voltage induced in the alternator is proportional to stroke and most electrical loads require a stable, constant voltage, such as 24 vdc or 28 vdc or 115 vac. Consequently, it is a problem to design a control system that accomplishes both (1) matching the power delivered to the alternator by the FPSE to the power demanded by the electrical load plus electrical losses; and (2) maintaining a constant output voltage. The problem is that, if the electrical power demand of the electrical load decreases and the stroke is decreased to reduce FPSE power, the induced voltage drops. Conversely, if an increased electrical power demand results in an increased stroke to provide more power from the FPSE, the output voltage also increases. It is therefore desirable to modulate the power from the FPSE to match electrical load power while reducing or eliminating voltage variations at the electrical load that result from variations in load power demand.
Additionally, there is a need for a manner of controlling piston stroke by a feedback control system that can more quickly detect variations in operating parameters resulting from system disturbances that lead to unwanted variations in piston stroke and that can quickly respond to the detected variations so that the actual piston stroke is maintained within tighter boundaries.
Similarly, there is a need for a manner of controlling piston stroke that can quickly vary the piston stroke in response to changes in the electrical power out load demand and maintain the balance of Stirling engine generated power out and power absorbed by the alternator.
Therefore, it is an object and feature of the present invention to provide an improved way of controlling piston stroke in order to match mechanical power produced by the engine to mechanical power absorbed from the engine by the alternator which is essentially the electrical power required by the user load.
A further object and feature of the invention is to control piston stroke based upon an operational parameter that can be more easily and more quickly controlled thereby permitting control of piston stroke, and therefore of power balance, within closer tolerances.
A further object and feature of the invention is to combine the piston stroke control with circuitry and a feedback control loop to provide improved voltage regulation so that the FPSE can be operated over a wide range of piston stroke for maintaining the balance of the engine power transferred to the alternator and yet still provide a relatively constant, well regulated output voltage to the electrical load over a wide range of load power consumption.
Yet another object and feature of the invention is to provide an even simpler, more stable and more effective controller for controlling an electrical power generating source comprising a free piston Stirling engine driving a linear alternator than previously disclosed.